The present invention relates to a spindle motor for use with a small sized memory device such as hard disc drive. Also, the present invention relates to a hard disc drive incorporating the spindle motor therein.
Typically, the small sized memory device such as hard disc drive has been required to have a large storage capacity and to allow a quick read and write operation from and into hard discs mounted in the hard disc drive. This in turn requires drive means or spindle motor in the memory device to be small, light, and highly reliable. For such requirements, the spindle motor should be designed so that, even if small sized, it can rotate the increased number of hard discs in an increased rotational frequency with a desired precision.
FIG. 1 shows an example of conventional spindle motors. The spindle motor includes a housing 1 or fixed base. A vertical shaft 2 is fixedly connected at one end thereof to the housing 1. A ball bearing 3, which is provided around the shaft 2, carries a rotor hub 4 for rotation around the shaft 2. A plurality of storage medium or hard discs 6 are supported around the rotor hub 4 in a coaxial fashion so that each of the hard discs 6 extends perpendicular to the shaft 2 and spaces a certain gap away from the neighboring hard disc or discs. A plurality of stators 7, each of which bears a coil wound therearound, are arranged at regular intervals along an outer periphery of a cylindrical portion formed in the housing 1 in a coaxial fashion with the shaft 2. On the other hand, a plurality of magnets 9 are arranged at regular intervals along an inner periphery of a cylindrical portion formed in the rotor hub 4, surrounding around the outer periphery and leaving a certain gap therebetween, so that the magnets. 9 confront to the stators 7.
In operation of the spindle motor so constructed, a certain alternating current is applied to the coils 8 to form an alternating magnetic field therearound, causing the rotor hub 4 and hard discs 6 supported thereby to rotate in a predetermined direction through the bearing 3 around the shaft 2. Although not shown, in order to read and write information from and into the hard discs, a plurality of access arms are positioned corresponding to respective hard discs. Each access arm carries a read and write head or magnetic head for reading information from the corresponding hard disc and writing information onto the same.
As described above, the hard disc drive is required not only to be small but also to read and write information in an increased rate. To this end, the hard disc drive should be provided with some improvements and modifications. Specifically, the shaft 2 of the hard disc drive shown in FIG. 1, supported only at its one end, has a typical outer diameter of about 6 mm and is made of stainless steel. Disadvantageously, the number of hard discs supported by such shaft 2 is limited to six because, if supporting more number of hard discs on the shaft, the shaft would cause greater vibrations due to an insufficient rigidity of the shaft, which results in another defect in read and write operations for the hard discs 6.
Contrary to this, the market requires the hard disc drive to have further storage capacity. In response to the requirement, ten or more hard discs should be mounted on the rotor hub. For the purpose, there have been proposed several techniques in the art. One technique is to increase the diameter of the shaft 2, and another is to support the shaft at its opposite ends, thereby reducing the vibrations of the shaft.
FIG. 2 shows another hard disc drive employing the latter technique in which the shaft is supported at its opposite ends. In this hard disc drive, the shaft 10 is formed at its top end portion with a threaded hole 11 extending along a longitudinal axis of the shaft 10. Also, the top end of the shaft 10 is connected with a hard disc cover 13 fixed on a base not shown. The connection between the shaft 2 and the hard disc cover 13 is performed with a screw bolt 14 which is inserted through a through hole 15 formed in the hard disc cover 13 and then screwed into the threaded hole 11 of the shaft 10. The shaft 10, so supported at its opposite ends and thereby provided with the increased rigidity, can support more hard discs, e.g., ten hard discs, and rotate them without any vibrations that would provide adverse affects for read and write operations.
Disadvantageously, the proposed two techniques have respective drawbacks that should be solved. For example, the former technique in which the diameter of the shaft is increased results in an increase of weight of the hard disc drive, deteriorating the compactness of the hard disc drive. The latter technique on the other hand requires additional processes for the formations of the holes 11 and 15. Also, the threaded hole 11 and the through hole 15 should be formed so that, when the hard disc cover 13 is assembled on the base, they position in a coaxial fashion, which needs a great precision for the machining of both holes.
Another technique has been employed for the spindle motor in order to meet the requirement of compactness thereof, in which a gas-lubricated or oil-lubricated hydrodynamic bearing is used for the bearing assembly in the spindle motor. Among these the oil-lubricated hydrodynamic bearing has a drawback that an increased rotational movement causes a high temperature in the bearing assembly due to the viscosity of the oil. This problem may be solved simply by decreasing the diameter and then the peripheral speed of the shaft, which in turn results in the above-mentioned problem caused by the reduced diameter of the shaft.
Therefore, an object of the present invention is to solve the above-mentioned problems of the conventional spindle motor. Another object of the present invention is to provide an improved spindle motor allowing a small sized hard disc drive to have an enlarged storage capacity. Another object of the present invention is to provide an improved spindle motor allowing the shaft to, rotate in a high speed in the hydrodynamic bearing assembly. Another object of the present invention is to provide an improved hard disc drive incorporating such spindle motor.
Accordingly, a spindle motor of the present invention includes a fixed base member, an outer cylindrical member made from a hollow tube, and an inner cylindrical member inserted in the outer cylindrical member. The inner and outer cylindrical members are arranged so that one of the outer and inner cylindrical member is fixed to the base member and the other is rotatable about a longitudinal axis thereof. A bearing assembly is provided for bearing the rotatable cylindrical member on the fixed cylindrical member. Also, drive means is provided for rotating the rotatable cylindrical member about the fixed Cylindrical member. In particular, the fixed cylindrical member is made of ceramic.